Phosphorylated cellulose ester membrane for use in separation or concentration of substances

ABSTRACT

A novel phosphorylated cellulose ester membrane containing 0.1 to 10 % of phosphorus and having a selective permeability. This permselective phosphorylated cellulose ester membrane consists essentially of a phosphorylated cellulose ester obtained by reacting a phosphorylating reagent such as phosphorus oxychloride with a lower aliphatic acid ester of cellulose, which has a reactive hydroxyl group, in an organic solvent, and then hydrolyzing the resulting ester. The membrane can be used widely in separating materials by processes such as reverse osmosis, ultrafiltration, dialysis, electrodialysis, and the like.

This invention relates to a novel phosphorylated cellulose estermembrane useful for the separation or concentration of substances.

An object of this invention is to provide a novel phosphorylatedcellulose ester membrane having a selective permeability, which can beused widely in separating materials by various processes such as reverseosmosis, ultrafiltration, dialysis, electrodialysis, and the like, andalso to provide the manufacturing method of said membrane.

The phosphorylated cellulose ester membrane of this invention consistsessentially of a phosphorylated cellulose ester obtained by reacting aphosphorylating reagent such as phosphorus oxychloride with a loweraliphatic acid ester of cellulose, which has a reactive hydroxyl group,in the form of an organic solvent solution, and then hydrolyzing theresulting ester. Membranes suitable for the above-said various processesare obtained by properly controlling the phosphorylation reaction of thecellulose ester, thereby to vary various factors such as phosphoruscontent, ion-exchange capacity, water content, etc., of the resultingphosphorylated cellulose ester.

For example, there may be adopted a method whereby phosphate groups areintroduced in a cellulose ester, which comprises reacting aphosphorylating reagent in the presence or absence of a suitable basewith a cellulose ester dissolved in a suitable solvent tochlorophosphonate the hydroxyl group. To go into some detail, acellulose ester is dissolved in a solvent, and reacted with aphosphorylating reagent and, if necessary, a base to remove hydrogenchloride formed during the reaction; the phosphorus content of theproduct may be controlled as desired by adjusting the factors such asconcentration of the cellulose ester in the solution, molar ratio of thehydroxyl group in the cellulose ester to the phosphorylating reagent,reaction temperature, etc. Concentration of the cellulose ester in thesolution is generally 0.1 to 35 %, preferably 1 to 25 %. Molar ratio ofthe hydroxyl group in cellulose ester to the phosphorylating reagent isfrom 0.1 to 1000, preferably 1 to 100. The reaction temperature rangesfrom -78°C. to the boiling point of the solvent employed, preferablyfrom 0 to 30°C. In order to obtain a membrane which is excellent in theperformance for separating materials for various purposes and retainssufficient strength while in use, it is desirable to control thereaction so that the phosphorus content of the hydrolyzate may become0.1 to 10 %.

Hydrolysis of the chlorophosphonated cellulose ester is easily effectedby simply treating the ester generally with water or a dilute aqueousalkali solution. The hydrolysis can be carried out by pouring thesolution containing the reaction mixture from the chlorophosphonationstep directly into water prior to the film-casting, or, alternatively,by preparing a film-casting solution composition directly from saidreaction solution, casting a film on a clean plate glass surface fromsaid composition, and, after a suitable period of time, immersing thecast film in water. A period of several minutes is generally sufficientfor the hydrolysis to be complete.

The cellulose esters to be used as the starting material for such amembrane can be any of those which have a reactive hydroxyl group.Examples of such esters include cellulose esters of lower aliphaticacids such as formic acid, acetic acid, propionic acid,β-hydroxypropionic acid, butyric acid, lactic acid, tartaric acid,pyruvic acid, malic acid, maleic acid, methacrylic acid, crotonic acid,sorbic acid, succinic acid, methylsuccinic acid, ethylsuccinic acid,propylsuccinic acid, and adipic acid, and mixed esters of these acids.Of the esters listed above, cellulose acetate and cellulose butyrate areparticularly preferred.

The desirable reagents for phosphorylating the cellulose ester arephosphorus oxychloride (POCl₃) and phosphorus oxybromide (POBr₃), thoughother phosphorylating reagents can be used without causing anyparticular trouble. Phosphorus oxychloride and phosphorus oxybromide aresuitable in view of mildness of the reaction, causing no decrease inmolecular weight of the cellulose ester.

The solvents to be used in dissolving the cellulose ester are methylacetate, acetone, methyl ethyl ketone, acetyl methyl cellulose,nitromethane, and chloroform. Of these, acetone is preferred.

The bases for use in removing hydrochloric acid are pyridine,diethylamine, and triethylamine. Of these, pyridine is preferred.

The phosphorylated cellulose ester thus obtained has the followingstructure: ##EQU1## where "Cell" represents cellulose structural unit,##EQU2## represents a lower aliphatic acid residue, and x, y, and z arepositive numbers satisfying the relation x + y + z = 3.

The permselectivity of the membrane is greatly affected by castingconditions, particularly the composition, evaporation rate, andevaporation temperature of the solvent used in the casting solution.Consequently, by varying these factors and by properly selecting, inaddition, the period of hydrolysis in the case of direct casting fromthe reaction solution, it is possible to govern, as desired, themolecular weight cut-off characteristics of the membranes and also tocontrol the size of pores in the membranes ranging from a symmetricmembrane to an asymmetric or porous membrane.

The symmetric membrane is commonly cast from a solution ofphosphorylated cellulose ester in a good solvent. Examples of such goodsolvents are formic acid, acetic acid, acetic anhydride,dimethylformamide, dimethyl sulfoxide, m-cresol, α-pyrrolidone, triethylphosphate, and acetone. Preferred among these are formic acid andacetone.

On the other hand, the asymmetric or porous membrane is formed when afilm is cast from the reaction mixture solution, which has been removedof precipitated hydrochloride, and, after having been freed from a partof the solvent by evaporation, the cast film is hydrolyzed by immersionin water. A more general procedure comprises casting a film from asolution of the phosphorylated cellulose ester in a solvent mixture ofthe aforesaid good solvent and a poor solvent having at least a higherboiling point than that of the good solvent, and, after removal of apart of the solvent by evaporation, treating the cast film by immersionin a coagulating liquor. The coagulating liquor should be a nonsolventfor the phosphorylated cellulose ester and, at the same time, a solventcompatible with both the good solvent and the poor solvent employed inthe casting solution. Examples of poor solvents are water, methanol,formamide, pyridine, of which water is preferred. Examples of thecoagulating liquors which can be used are water, alcohols, and ketones.

The casting solution which gives a membrane of asymmetric structure isprepared by adding 5 to 50 % by volume of a poor solvent to a goodsolvent containing a phosphorylated cellulose ester. The time ofimmersion of the cast film, after partial evaporation of the solvent, ina coagulating liquor should be sufficient for the membrane to developthe structure, which usually requires one minute to several hours. Theperiod of solvent evaporation determines the surface structure ofmembrane. When the period is long an asymmetric membrane having a thinhigh-density surface layer is obtained, while when the period is short aporous membrane having no such high-density surface layer is obtained.low-molecular-weight

Although there exists, of course, no sharp border line, there is used,in general, an asymmetric membrane in the electrodialysis where waterpermeability is not required, a thin symmetric membrane or an asymmetricmembrane having a thin high-density surface layer in the reverse osmosiswhere a high water-permeability as well as a solute impermeability tothe solute are required, and either a porous membrane or a symmetricmembrane in the ultrafiltration or dialysis where permeabilities of bothwater and a low-molecular-weigh solute are required.

The membrane thickness can be varied over a wide range by varying thepolymer concentration in a casting solution and the clearance of anapplicator, but a thickness of 1 to 500 μ is suitable for the membraneto maintain sufficient mechanical strengths and practically usefulphysical properties. The casting plate generally emmloyed in membranepreparation is plate glass, metal beltings, or the like. It is alsopossible to cast a membrane on woven or nonwoven backing material ofnatural or synthetic fibers of the purpose of reinforcing the membrane.By use of a reinforcing material in the proper form, it is feasible toproduce membranes of various shapes, particularly of a plate type or atubular type.

It is one of the characteristic features of the present phosphorylatedcellulose ester membrane that the permselectivity of the membranedepends on its phosphorus content.

In reverse osmosis, the dissolution process of water into the membranehas an important meaning. When the phosphate content falls within therange of 1 to 10 %, water permeability of the present phosphorylatedcellulose ester membrane is so high due to hydrophilic property of thephosphate group that the membrane is best suited for use in the reverseosmosis. It is particularly desirable to use the present phosphorylatedcellulose ester membrane in separating inorganic salts by reverseosmosis, because the membrane is a charged membrane havingcation-exchange property, and hence, exhibits a higher inherent abilityto exclude anions in the feed solution, as compared with usual unchargedmembranes when used in the material-separating process. The presentmembrane shows no change in membrane performances when stored dry, incontrast to commercial cellulose acetate membranes which, once dried,show a markedly deteriorated performance that cannot be restored to theoriginal level.

For a membrane to be used in ultrafiltration and dialysis, a highselective permeability to the solute is an absolutely necessarycondition. To be suitable for such a membrane, phosphorous content ofthe phosphorylated cellulose ester material is desirably in the rangefrom 3 to 10 %. In the said range, no loss is found in mechanicalstrengths of the membrane. When used as dialyzing membrane for anartificial kidney, the present membrane also gives favorable resultswith respect to anticoagulation of blood, owing to the phosphate groupin a side chain.

In order to obtain an ion-exchange membrane having an optimumcombination of the selective permeability and the electric conductivity,it is desirable to prepare a membrane containing phosphate groupscorresponding to an ion-exchange capacity of 0.1 to 3.5, preferably 1 to2, meq/g (meq/g means milliequivalent per gram of the membrane). Such amembrane has an electric resistance per unit surface area of 5 to 11Ω.cm² and can be used as an ion-exchange membrane for use in ordinaryelectrodialysis.

Since the present phosphorylated cellulose ester membrane has suchcharacteristic properties as mentioned in the foregoing, it can bewidely utilized in a unit operation for separating materials. Forexample, components which can be separated from aqueous solutions byreverse osmosis include inorganic salts comprising anions such asfluoride ion, bromide ion, chloride ion, nitrate ion, sulfate ion,phosphate ion, chromate ion, borate ion, and carbonate ion, and cationssuch as sodium ion, potassium ion, magnesium ion, calcium ion, ferrousion, ferric ion, and barium ion; organic compounds such as alcohol,phenols, amines, and carboxylic acids; viruses, bacteria, proteins, andother natural as well as synthetic polymers. Practical examples forseparating such materials are desalination and purification of salinewater, brackish water, and general waste water; softening of hard water,desalination of sea water, and recovery of useful substances from seawater. The present invention membrane is also suitable for concentrationof fruit juice, vegetable juice, molasses, milk, coffee extract, andmany other materials. It is particularly suited for use in purifyingaqueous solutions containing dissolved inorganic salts.

Further, the present phosphorylated cellulose ester membrane is alsosuited for use in ultrafiltration and dialysis to separatelow-molecular-weight solutes from high-molecular-weight solutes andcolloids, and in electrodialysis as an ion-exchange membrane to separatevarious substances.

The invention is illustrated below in detail with reference to Examples,but the invention is not limited to the Examples.

EXAMPLE 1

In a 500 four-necked flask equipped with a thermometer, stirrer,dropping funnel, and nitrogen inlet, were placed 50 g of a celluloseacetate (reagent grade, Eastman Organic Chemicals Co., 39.8 % acetylcontent) and 75 g of phosphorus oxychloride. After addition of 375 g ofacetone to dissolve the reactants in the flask, 50 g of pyridine wasadded dropwise under a steam of nitrogen at 20° to 25°C, and thereaction was allowed to proceed with stirring for 3 hours. The reactionproduct was isolated by pouring into a large volume of water, repeatedlywashed with water until the washings no longer showed acidity, and driedthoroughly. The product thus obtained was a phosphorylated celluloseacetate having a phosphorus content of 1.80 % and an ion-exchangecapacity of 1.00 meq./g (based on dry volume of membrane).

A 20 % solution of the said phosphorylated cellulose acetate, indimethylformamide was prepared and poured on a clean plate glass to forma membrane. After having been dried at room temperature for 18 hours,the membrane had a thickness of 59 μ and a water content of 38.7 % onwet weight basis.

The membrane was subjected to reverse osmosis membrane test using a0.5-% aqueous solution of sodium chloride. The solution was fed under apressure of 91 atmospheres and at a rate of 270 cc/minute to a 5-cc cellwhich held the membrane, 47 mm in diameter. The phosphorylated celluloseacetate membrae showed 91 % rejection against sodium chloride and awater flux of 1.2 gallons/foot².day (hereinafter abbreviated to gfd).

EXAMPLE 2

Phosphorylation of cellulose acetate was conducted in the same way as inExample 1, except that 1,000 g of acetone was used to dissolve thecellulose acetate. There was obtained a product having a phosphoruscontent of 4.06 % and an ion-exchange capacity of 2.08 meq/g.

A 10-% solution of the said phosphorylated cellulose acetate in formicacid was prepared and poured on a clean plate glass to form a membrane.After having been dried at room temperature for 25 hours, the resultingmembrane having a thickness of 15 μ, showed 93.7 % rejection againstsodium chloride, and a water flux of 3.1 gfd, as measured in the samemanner as in Example 1.

EXAMPLE 3

A 15-% solution of the phosphorylated cellulose acetate obtained inExample 1 was prepared by use of a tetrahydrofuran-methanol (3 : 1 byvolume) mixture, and poured on a clean plate glass to form a membrane.After having been dried at room temperature for 20 hours, a transparentmembrane, 20 μ in thickness, was obtained.

The membrane was subjected to reverse osmosis membrane test using a0.5-% aqueous sugar solution. The solution was fed under a pressure of15 atmospheres and at a rate of 270 cc/minute to a 5-cc cell which heldthe membrane, 47 mm in diameter. The membrane showed 82 % rejectionagainst sugar and a water flux of 13.5 gfd.

EXAMPLE 4

In a manner similar to that in Example 1, 20 g of cellulose acetate wasdissolved in 150 g of acetone. After addition of 15 g of phosphorusoxychloride, 10 g of pyridine was added dropwise under a stream ofnitrogen and at 20° to 25°C. The reaction was allowed to proceed for 3hours with stirring.

After completion of the reaction, the reaction solution was directlypoured on a clean plate glass and the solvent was evaporated for 60seconds at room temperature. Immediately thereafter, the cast filmtogether with the plate glass was immersed in water at 5° to 6°C, andhydrolysis and extraction of the solvent were continued for 3 hours toobtain a white opaque membrane, 80 μ in thickness.

The membrane was subjected to ultrafiltration membrane test using a0.25-% aqueous solution of trypsin (molecular weight 20,000). Thetrypsin solution was fed under a pressure of 10 atmospheres and at arate of 270 cc/minute to a 5-cc cell which held the membrane, 47 mm indiameter. The membrane showed 100 % rejection against trypsin and awater flux of 50 gfd. Phosphorus content of the membrane was 1.84 %.

EXAMPLE 5

In a manner similar to that in Example 1, 20 g of cellulose acetate wasdissolved in 150 g of acetone. After addition of 7.5 g of phosphorusoxychloride, 5 g of pyridine was added dropwise under a stream ofnitrogen and at 20° to 25°C. The reaction was allowed to proceed for 3hours with stirring. The reaction product was isolated by pouring into alarge volume of water, repeatedly washed with water until the washingsno longer showed acidity, and dried thoroughly to obtain a product whichwas phosphorylated cellulose acetate having a phosporus content of 0.73%.

A 30-% solution of the said phosphorylated cellulose acetate in formicacid was prepared and poured on a clean plate glass to form a membrane.After having been dried at room temperature for 30 hours, the membraneshowed a thickness of 90 μ, an ion-exchange capacity of 1.04 meq/g, andan electric resistance of 10.1 Ω.cm², as measured in a 0.5-N aqueoussolution of sodium chloride.

EXAMPLE 6

In a manner similar to that in Example 1, 50 g of cellulose acetate wasdissolved in 800 g of acetone. After addition of 15.0 g of phosphorusoxychloride, 10 g of pyridine was added dropwise under a stream ofnitrogen and at 20° to 25°C. The reaction was allowed to proceed for 4hours with stirring. The reaction product was isolated by pouring thereaction mixture in a large volume of water, repeatedly washed withwater until the washings no longer showed acidity, and dried thoroughlyunder reduced pressure to obtain a phosphorylated cellulose acetatehaving a phosphorus content of 0.22 %.

A 10-% solution of the said phosphorylated cellulose acetate in acetonewas prepared and poured on a clean plate glass to form a membrane. Afterhaving been dried at room temperature for 5 hours, the membrane showed athickness of 10 μ. Desalination performance of the membrane was measuredin a manner similar to that in Example 1 and found that under a pressureof 50 atmospheres the rejection against sodium chloride was 99 % and thewater flux was 0.62 gfd.

EXAMPLE 7

Phosphorylation of cellulose acetate was conducted in the same manner asin Example 6, except that 52.5 g of phosphorus oxychloride and 35 g ofpyridine were used. A product containing 0.80 % of phosphorus wasobtained.

A 10-% solution of the said phosphorylated cellulose acetate indimethylformamide was prepared and poured on a clean plate glass to forma membrane. After having been dried at room temperature for 30 hours,the membrane showed a thickness of 8 μ.

Desalination performance of the membrane was measured in a mannersimilar to that in Example 1. It was found that under a pressure of 50atmospheres the rejection against sodium chloride was 97.5 % and thewater flux was 0.75 gfd.

EXAMPLE 8

Phosphorylation of cellulose acetate was conducted in the same manner asin Example 6, except that 67.5 g of phosphorus oxychloride and 45 g ofpyridine were used. A product containing 1.40 g of phosphorus wasobtained.

A 10-% solution of the said phosphorylated cellulose acetate indimethylformamide was prepared and poured on a clean plate glass to forma membrane. After having been dried at room temperature for 30 hours,the membrane showed a thickness of 11 μ.

Desalination performance of the membrane was measured in a mannersimilar to that in Example 1. The membrane showed under a pressure of 50atmospheres 90.0 % rejection against sodium chloride and a water flux of0.97 gfd.

What is claimed is:
 1. A phosphorylated cellulose ester membrane solublein solvent for use in separation or concentration of substances in anaqueous solution, having a phosphorus content of from 0.1 to 10% and theformula ##EQU3## wherein Cell is a cellulose structural unit, ##EQU4##is a lower aliphatic acid residue, and x, y and z are positive numberssatisfying the relation x+y+z=3.
 2. A phosphorylated cellulose estermembrane according to claim 1, wherein the membrane is a symmetricmembrane.
 3. A phosphorylated cellulose ester membrane according toclaim 1, wherein the membrane is an asymmetric membrane.
 4. Aphosphorylated cellulose ester membrane according to claim 1, whereinthe membrane is a porous membrane.
 5. A phosphorylated cellulose estermembrane according to claim 1, wherein the membrane has a thickness of 1to 500 μ.
 6. A phosphorylated cellulose ester membrane according toclaim 1, wherein phosphorus content of the membrane is in the range from1 to 10 %.
 7. A phosphorylated cellulose ester membrane according toclaim 1, wherein phosphorus content of the membrane is in the range from3 to 10 %.
 8. A phosphorylated cellulose ester membrane according toclaim 1, wherein ion-exchange capacity of the membrane is 0.1 to 3.5meq./g.
 9. A phosphorylated cellulose ester membrane according to claim1, wherein the cellulose ester is at least one member selected fromlower aliphatic acid esters of cellulose.
 10. A phosphorylated celluloseester membrane according to claim 9, wherein the lower aliphatic acidester of cellulose is an ester of cellulose with formic acid, aceticacid, propionic acid, β-hydroxypropionic acid, butyric acid, lacticacid, tartaric acid, pyruvic acid, malic acid, maleic acid, methacrylicacid, crotonic acid, sorbic acid, succinic acid, methylsuccinic acid,ethylsuccinic acid, propylsuccinic acid, or adipic acid.
 11. Aphosphorylated cellulose ester membrane according to claim 10, whereinthe lower aliphatic acid ester of cellulose is cellulose acetate orcellulose butyrate.